Characterization of core neutrals using passive measurements of the D α spectrum near the X -point in the DIII-D tokamak

Abstract New spectroscopic measurements of deuterium Balmer- α emission are used to infer the spatial distribution of neutral particles near the X -point in a diverted high confinement mode plasma. The charge exchange neutral spectroscopy (CENS) diagnostic—recently installed on the DIII-D tokamak—uses 15 lines of sight extending from the edge of the confined region ( ρ ≈ 0.7) to the X -point in lower single-null plasmas. Each CENS line of sight is spectrally resolved around the deuterium Balmer- α wavelength (6561 Å) to measure the Doppler shift, line broadening, and intensity of D α emission. Thermal line broadening is used to identify emission from neutrals undergoing charge-exchange (CX) with high temperature ions in the confined plasma. This spectral information allows a more accurate determination of the neutral density deeper inside the confined plasma compared to traditional filter-based diagnostics. We present two methods of analyzing CENS measurements: (1) fitting the spectrum from each view-chord separately, and (2) a tomographic method for inverting the neutral density over a 2D region of space using all CENS views collectively. The neutral density profile is found to decay exponentially in the radial direction with two scale-lengths. In the pedestal region the neutral density decays at a rate approximately equal to the local mean-free-path for CX collisions, L n D 0 ≈ λ CX . Further inside the plasma the neutral density decays at a rate equal to the mean-free-path for ionization, L n D 0 ≈ λ inz. . The separatrix n D 0 value is found to be approximately 2 × 10 15 m −3 . Based on results from the 2D inversion, the density of neutrals is found to increase along the separatrix approaching the X -point by a factor of ≈ 5 × over the region covered by the CENS diagnostic.